Trust is the "assured reliance on the character, ability, strength, or truth of someone," or "one in whom confidence is placed." I think primarily truth or honesty of the other person.

I suppose I trust friends and family members. Also, I hope if I make a mistake that I will be given the opportunity to correct the mistake or make amends.

I trust friends to be honest with me.

Obviously I trust certain people whom I've known and to whom I have entrusted myself.

Loss of trust can be annoying, dispiriting to downright devastating, e.g. when one spouse finds that the other has been unfaithful.

I think some time is required for complete trust to be established - it comes with experience.

Why trust others? Because to live without trusting anyone would be a sad existence.

I think it's kinda funny how we rationalists define things using more definitions. And then I remember that it's so much more complex than that.

Well, there are two solid ways to define it, that I can see:

Through biochemistry and neurology: the chemical process in the brain that make up the reactions necessary to experience "trust."

Through the experiential feelings that humans have that make up trust, and what it means to them.

Using the second method --because I suck at chemistry 101-- I defined it thusly:

Trust is the idea that roughly stated means to accept a nearly complete free-flow of information between two or more individuals, or the allowance of certain actions, with the expectation of a non-negative effect(s) to occur.

Who or what do I trust? I trust logic, the scientific method, and the person I will one day choose to live with.